I guess they are all 'agent markers'. As people obviously don't say 'actists' for actors so I guess they are not interchangeable. But where are the differences?
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Here are a few guidelines for these suffixes:
orare both suffixes used to turn a verb into a noun by referring to the person/agent responsible for the action. The one rule I could find for the distinction between
-oris that Latin-based words tend to use
-or. Otherwise, it's more common to use
-erinstead. This can refer to any agent (person, thing, event etc) that does the action. For example, a mixer is something that blends (mixes) other things together. It can be an electric kitchen device to aid mixing ingredients; it can be the person whose duty it is to mix stuff; it can even be an event whose purpose is to get a bunch of strangers to "blend together" and interact.
-eesuffix is used to mean a person who is the target/recipient of an action. So for example, the trainee is a person receiving training. Compare this to the trainer, who is the person in charge of trainer the new trainee.
-istrefers to specifically a person who does something or is associated with something. Non-person agents are not typical of this suffix.